It was one team splitting half-way and taking everything from the other.
IBM went their own way with OS/2 and Microsoft hired Dave Cutler from Digital to develop NT over several years. Windows NT is not OS/2. It never was.
Microsoft unilaterally changed the OS/2 3.0 API to the match the Windows API, IBM did not approve of that, and then the project split, with the Microsoft version of OS/2 3.0 becoming Windows NT.
IBM went their own way with OS/2 and Microsoft hired Dave Cutler from Digital to develop NT over several years. Windows NT is not OS/2. It never was.